The present invention relates to spectacle lens production, and more particularly to the use of single-point lens blocking for surfacing and edging of sphero-cylindrical (toroidal) lens surfaces.
Spectacles (framed eyeglasses) commonly utilize lenses having a convex outer surface and a concave inner surface having one or more different radii of curvature. In the simplest form of such lenses, both the inner and outer surfaces are spherical. However for many users the inner surface is in the shape of a section of a torus, commonly referred to as a sphero-cylindrical surface, since the surface is formed by generating two curved surfaces of different radii at right angles to one another. The direction along which the longer radius is generated is referred to as the cylinder axis of the lens. Regardless of type, such lenses are produced from a glass or plastic blank by generating the inside surface according to a prescription by first roughly cutting the desired curved shape and thereafter performing one or more stages of abrasive fining and subsequently one or more stages of polishing. Thereafter the peripheral edge of the lens is cut to a shape to match the desired frame. Preparatory to the surfacing and edging processes, respective blocks are adhered to the outer lens surface in respective positions to provide a means of holding the lens for the particular operation.
An eyeglass prescription for a typical lens specifies, among other things, the refractive power to be produced by the lens or the respective sphere and cylinder powers in the case of a sphero-cylindrical lens, the location of the cylinder axis relative to the frame for a sphero-cylindrical lens (along which the sphere power is generated), and the decentration, that is the distance and direction (usually horizontal) from the center of each frame to the corresponding optical center of the lens to be placed therein, in terms of both the left and the right eye. In some cases the prescription will also specify a particular prism, that is, a tilt in a predetermined direction of the lens surface relative to the frame, and the location of lens segments relative to the frame for multifocal lenses.
In the conventional production of a spectacle lens, points on the outer surface of an appropriate lens blank are selected to correspond to the frame center and optical center of the lens respectively. Typically the selected frame center point is the center of the blank, except in multifocal lenses where the position of the segment relative to the frame has been specified, thereby dictating some other position for the frame center. Once the frame center is selected, the optical center for the lens may be determined by moving a distance from the frame center corresponding to the decentration prescribed. A lens holding block is then normally attached by commonly known adhesion methods to the outer lens surface with its center corresponding to the optical center of the lens so as to provide a means of holding the lens while the interior surface is generated by an appropriate cutting tool of any conventional type well known to the art. After the rough surface is cut one or more grinding and polishing steps are performed to finish the surfacing. The block is then removed and the lens is cleaned and checked for accuracy using a lensometer. Subsequently, the lens is again blocked, this time with the center of the block corresponding to the frame center, and the lens is placed in a machine for producing the edge configuration required for mounting the lens in the specified frame. When the edging is completed the second block is removed and the lens is cleaned and mounted in the frame.
The aforementioned conventional process for producing lenses requires complex dual positioning of lens blocks and thereby often leads to positioning errors causing the lens to be discarded and redone. More important, the mounting of the lens block at the optical center of the lens blank preparatory to surface generation can result in an uneven distribution of force during the fining and polishing steps which produces uneven surfacing and concomitant unwanted tilting and distortion, requiring costly further surfacing steps to correct. This adverse result is caused by the fact that the prescribed optical center of the lens is usually far enough from the center of mass of the lens blank to produce substantial imbalance when the block, located at the optical center, is gripped for fining and polishing. This imbalance cannot effectively be corrected by "cribbing" or cutting away excess lens blank material around the edges because the later relocation of the block to the frame center of the lens for edging will require the use of much of the excess material in the ultimate formation of the edge.
Moreover the conventional dual attachment and detachment of the lens blocks increases the likelihood of lens breakage, and in any event requires costly extra steps.
A system has previously been developed employing an adjustable, multi-piece block structure which requires only a single attachment of the block to the lens blank for both the surface generation and edging processes. However such process requires time-consuming adjustment of the relative positions of the multiple pieces of the block and, more important, conventionally utilizes the optical center for surface generation and the frame center for edging, thereby failing to cure the aforementioned problems of error incident to multi-point blocking and imbalance and uneven surfacing due to optical-center blocking during fining and polishing.
It has heretofore also been known that frame-centered surfacing of lenses can be accomplished in the production of a simple spherical surface by the application of "prism," that is by tilting a lens blank, which is blocked at the frame center rather than optical center, relative to the cutting tool by means of a tapered spacer known as a "prism ring" such that the optical center will be produced with the correct amount and direction of decentration. However it has not previously been known how to utilize the "prism" technique in correctly decentering the optical center of sphero-cylindrical surfaces. Accordingly use of frame-centered blocking during surfacing of lenses has not been feasible for the mass production of spectacle lenses because uniformity of technique for both spherical surfaces and sphero-cylindrical surfaces is necessary.
Accordingly there is a need for a system of surfacing and edging spectacle lenses applicable both to spherical and sphero-cylindrical lenses which does not require blocking at two separate centers for the surfacing and edging processes, but rather permits blocking only at the frame center of the lens blank for both processes, whereby imbalance and uneven force application during the surfacing process can be minimized and cribbing may be more effectively utilized to eliminate unneeded glass and its associated imbalance. Moreover such process should preferably require only a single block attachment step, at the frame center, to accomplish both processes and thereby eliminate costly steps and breakage.